The invention relates generally to power supplies, and, more specifically, to regulated power supplies using switched resistors having parasitic inductance.
One particular application for regulated power supplies is supplying power to ion lasers. Important factors for such power supplies are weight, speed, simplicity, reliability and radio frequency emissions.
U.S. patent application Ser. No. 754,036, filed July 11, 1985, describes a dissipative switched resistor regulator in which current flow through a resistor which is electrically connected to a load is controlled by varying the duty cycle of a switch which switches the resistor into and out of the circuit between a power source and the load; the resistor may be in series with or in parallel with the load. However, since there is always a series resistance between the source and the load when the switch is closed (for the series switched resistor regulator configurations), there is a minimum voltage drop which cannot be avoided even when the duty cycle is 100%. This unwanted lost power in the switched resistor results in poor efficiency. Also, the full source voltage cannot be applied to the load because of the voltage drop across the switched resistor when the duty cycle is 100%. The problem cannot be solved merely by decreasing the series resistance since this results in increased peak current through the switch which may exceed the capability of the switch to dissipate power. Also, decreased switched resistor resistance results in increased capacitor ripple currents.
The related U.S. patent application entitled, "Switched Resistor Regulator with Linear Dissipative Regulator", Ser. No. 945,924, filed Dec. 23, 1986 teaches a solution to this problem. The solution taught there is a linear dissipative regulator which bypasses the resistor and switch combination with a low resistance path when the duty cycle of the switch is 100%. This minimizes the power lost in the switched resistor during intervals when the duty cycle is 100%.
It has been found that the wire-wound resistors used for the switched resistor in some switched resistor regulator power supplies have parasitic inductance. This parasitic inductance causes voltage transients when current through the resistor is cut off. These voltage transients take the form of large voltage spikes which can damage switching transistors used to switch the resistors in and out of the circuit. Unless these voltage spikes are "snubbed", they can damage the switching transistors.
Designs for snubbers are well known. A simple design for a snubber is a capacitor which shunts the switching transistor. The capacitor acts as an initial short in passing the current of the voltage transient around the switch until the capacitor charges up. Such a design has the problem that the snubber capacitor charges up with the current in the transient spike. This charge must be bled off the snubber capacitor before the next cycle of the switch or the snubber capacitor will not be effective to snub the next transient. This bleeding off of charge occurs through the switch the next time the switch is turned on. This increases the electrical stress on the switch.
A lone capacitor cannot be used for the snubber without an additional series resistor. This is because the capacitor and the parasitic inductance together act as a tuned circuit and can oscillate or "ring". The addition of a series resistance sufficient to critically damp the circuit is necessary to suppress this ringing. However, this additional series resistance also slows down the process of charging and discharging the snubber capacitor. The slower charging tends to mitigate somewhat the effectiveness of the snubber. To regain the snubbing effectiveness, a diode is placed in parallel with the resistor such that the voltage transient will be of the proper polarity to forward bias the diode and charge the snubber capacitor through the diode. To protect the diode from being destroyed by excessive current, a small current-limiting resistor is placed in series with the diode. However, the diode is reverse biased during the discharge cycle to ready the snubber capacitor for the next transient, so discharging must occur through the series resistor. This slows the discharge rate and places a limitation on the maximum rate at which the switch may be operated since the next cycle cannot start until the snubber capacitor is fully discharged. A further disadvantage of the above-described prior art snubber design is that it is too complicated and expensive since it involves many components and many interconnections between them.
Accordingly, a need has arisen for a snubber design which is simple, effective, and inexpensive.